1. Field
One embodiment of the present invention relates to a method of driving a fuel cell device for supplying a current to an electronic device or the like.
2. Description of the Related Art
Presently, secondary batteries, such as lithium ion batteries, are mainly used as energy sources for electronic devices, e.g., portable notebook personal computers (notebook PCs), mobile devices, etc. In recent years, small, high-output fuel cells that require no charging have been expected as new energy sources to meet the demands for increased energy consumption and prolonged use of these electronic devices with higher functions. Among various types of fuel cells, direct methanol fuel cells (DMFCs) that use a methanol solution as their fuel, in particular, enable easier handling of the fuel and a simpler system configuration, as compared with fuel cells that use hydrogen as their fuel. Thus, the DMFCs are noticeable energy sources for the electronic devices.
Usually, a DMFC is provided with a fuel tank that contains methanol, a liquid pump that force-feeds the methanol to an electromotive section, an air pump that supplies air to the electromotive section, etc. The electromotive section is provided with a cell stack composed of laminated single cells, each including an anode and a cathode. As the methanol and air are supplied to the anode and cathode sides, respectively, electricity is generated by a chemical reaction. As reaction products that are produced by the electricity generation, unreacted methanol and carbon dioxide are generated on the anode side of the electromotive section, and water on the cathode side. The water as a reaction product is reduced to steam and discharged.
The fuel cell constructed in this manner has been developed as a cell that ensures a clean exhaust gas. In case of a system abnormality, unreacted methanol, excessive carbon dioxide, or intermediate products, such as formic acid, formaldehyde, etc., may possibly be discharged. In order to operate the fuel cell stably, therefore, its generated electricity and the temperature of the cell stack should be measured as the electromotive section is supplied with a fuel with an optimally controlled concentration.
A fuel cell system disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2006-286239, for example, is provided with a concentration/voltage control mechanism that lowers the concentration of an aqueous methanol solution and the voltage of an electricity generator of a DMFC as the temperature of the generator increases. There is proposed a method in which methanol is supplied to compensate for a shortage compared with a set concentration and the amount of methanol to be replenished is corrected by estimating a methanol consumption in a past predetermined period from a stack output and a fixed fuel consumption factor.
In general, however, the efficiency of electricity generation of a DMFC has characteristics such that it is not fixed with respect to changes in the cell stack temperature and a load current, takes its maximum at a certain temperature or load current, and decreases with distance from the maximum. If any conditions of a DMFC system, especially the stack temperature and the load current, vary when the DMFC system is activated or due to fluctuations in the ambient environment, variations in the energy consumption of an electronic device, etc., therefore, the conventional concentration control with the fuel consumption factor or the electricity generation efficiency regarded as fixed renders the concentration of the aqueous methanol solution that circulates through the anode unstable and affects the stability of the DMFC system.